Merger of a low voltage or low energy electron beam or flow into an energy recovery linac (ERL) apparatus may require a magnetic array to deflect the low energy electron beam onto a main ERL loop presenting a high energy electron flow. For a high average current ERL (in this context we may regard high current as 10 mA or greater average current), the low energy electron beam may comprise a bunch train of relatively high charge micro-pulses (for high average current, ERL beam charge may be typically in the nanocoulomb range). In the process of deflecting the low energy electron beam, space charge forces and natural energy spread in the electron beam may cause emittance growth in the electron beam as it goes through the merger magnetic array. In general, significant emittance growth in the merger may degrade the performance of the ERL.
Existing solutions for the required magnetic array may include constant angle three bend (e.g. left-right-left) chicanes, constant angle four bend (left-right-right-left) chicanes, constant angle four bend (left-right-left-right) dog-leg chicanes, and Z-bend (left-right-left-right—but with the two middle bends twice the bend angle of the outer bends).
The inventors have evaluated several electron merge solutions. For relatively high micropulse charge (in the nanocoulomb range) electron beams the various constant angle bend solutions lead to significant undesirable emittance growth (emittance growth of a few mm-mr may typically be considered to be significant for ERLs). The units “mm-mr” are “millimeter-milliradians”. Emittance entering the ERL may be significantly higher than the emittance for the electron source itself.
There is a need for an apparatus and method for merging a low energy electron flow into a high energy electron flow that has low emittance growth for high charge beams to enable the acceleration of high brightness, high current beams.